U.S. patent application number 17/100723 was filed with the patent office on 2022-05-26 for iral as a non-magnetic spacer layer for formation of synthetic anti-ferromagnets (saf) with heusler compounds.
The applicant listed for this patent is INTERNATIONAL BUSINESS MACHINES CORPORATION, Samsung Electronics Co., Ltd.. Invention is credited to Yari Ferrante, Panagiotis Charilaos Filippou, Chirag Garg, Jaewoo Jeong, Stuart Stephen Papworth Parkin, Mahesh Samant.
Application Number | 20220165938 17/100723 |
Document ID | / |
Family ID | |
Filed Date | 2022-05-26 |
United States Patent
Application |
20220165938 |
Kind Code |
A1 |
Jeong; Jaewoo ; et
al. |
May 26, 2022 |
IrAl AS A NON-MAGNETIC SPACER LAYER FOR FORMATION OF SYNTHETIC
ANTI-FERROMAGNETS (SAF) WITH HEUSLER COMPOUNDS
Abstract
A device including a first magnetic layer, a templating
structure and a second magnetic layer is described. The templating
structure is on the first magnetic layer. The second magnetic layer
is on the templating structure. The templating structure includes D
and E. A ratio of D to E is represented by D.sub.1-xE.sub.x, with x
being at least 0.4 and not more than 0.6. E includes a main
constituent. The main constituent includes at least one of Al, Ga,
and Ge. E includes at least fifty atomic percent of the main
constituent. D includes at least one constituent that includes Ir.
D includes at least 50 atomic percent of the at least one
constituent. The templating structure is nonmagnetic at room
temperature. At least one of the first magnetic layer and the
second magnetic layer includes at least one of a Heusler compound
and an L1.sub.0 compound.
Inventors: |
Jeong; Jaewoo; (San Jose,
CA) ; Filippou; Panagiotis Charilaos; (San Jose,
CA) ; Ferrante; Yari; (San Jose, CA) ; Garg;
Chirag; (San Jose, CA) ; Parkin; Stuart Stephen
Papworth; (San Jose, CA) ; Samant; Mahesh;
(San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Samsung Electronics Co., Ltd.
INTERNATIONAL BUSINESS MACHINES CORPORATION |
Gyeonggi-do
Armonk |
NY |
KR
US |
|
|
Appl. No.: |
17/100723 |
Filed: |
November 20, 2020 |
International
Class: |
H01L 43/10 20060101
H01L043/10; H01L 27/22 20060101 H01L027/22; H01L 43/02 20060101
H01L043/02; H01L 43/12 20060101 H01L043/12; G11C 11/16 20060101
G11C011/16 |
Claims
1. A device, comprising: a first magnetic layer; a templating
structure on the first magnetic layer, the templating structure
including D s and E, a ratio of D to E being represented by
D.sub.1-xE.sub.x, with x being at least 0.4 and not more than 0.6,
E including a main constituent, the main constituent including at
least one of Al, Ga, and Ge, E including at least fifty atomic
percent of the main constituent, D including at least one
constituent that includes Ir, D including at least 50 atomic
percent of the at least one constituent, the templating structure
being nonmagnetic at room temperature; and a second magnetic layer
on the templating structure, at least one of the first magnetic
layer and the second magnetic layer including at least one of a
Heusler compound and an L1.sub.0 compound.
2. The device of claim 1, wherein the templating structure includes
at least one layer of D is and at least one layer of E, the at
least one layer of E sharing an interface with the at least one
layer of D.
3. The device of claim 1, wherein xis at least 0.47 and not more
than 0.54.
4. The device of claim 1, wherein the at least one of the first
magnetic layer and the second magnetic layer has a thickness of not
more than five nanometers.
5. The device of claim 1, wherein D includes at least one of Ir and
IrRu.
6. The device of claim 1, further comprising: an additional
templating structure underlying the first magnetic layer, the
additional templating structure including D' and E', an additional
ratio of D' to E' being represented by D.sub.1-x'E.sub.x', with x'
being at least 0.4 and not more than 0.6, E' including an
additional main constituent, the additional main constituent
including at least one of Al, Ga, and Ge, E' including at least
fifty atomic percent of the main constituent, D' including at least
one additional constituent that includes at least one of Ir, Co and
Ru, D' including at least 50 atomic percent of the at least one
additional constituent, the additional templating structure being
nonmagnetic at room temperature.
7. The device of claim 1, wherein E includes at least one of Al, an
AlGe alloy, and an AlGa alloy.
8. The device of claim 7, wherein E is selected from Al, AlSn,
AlGe, AlGaGe, AlGaSn, to AlGeSn, and AlGaGeSn.
9. The device of claim 1, wherein the at least one of the first
magnetic layer and the second magnetic layer includes at least one
of Mn.sub.3.1-yGe, Mn.sub.3.1-ySn, Mn.sub.3.1-zSb,
Mn.sub.3.1-sCo.sub.1.1-tSn, a MnGa alloy, a MnAl alloy, an FeAl
alloy, a MnGe alloy, a MnSb alloy, and a MnSn alloy, with y being
is at least zero and not more than 0.6, z being at least 0 and not
more than 1.1, and with s being greater than zero and not more than
1.2 and t is greater than zero and not more than 1.0.
10. The device of claim 1, further comprising: an additional
magnetic layer; and a tunneling barrier layer between the
additional magnetic layer and the magnetic layer.
11. The device of claim 1, wherein the templating structure has a
thickness of at least three Angstroms and not more than eleven
Angstroms.
12. A device, comprising: a plurality of memory elements, each of
the plurality of memory elements including a first magnetic layer;
a templating structure on the first magnetic layer, including D and
E, a ratio of D to E being represented by D.sub.1-xE.sub.x, with x
being at least 0.47 and not more than 0.54, E including a main
constituent, the main constituent including at least one of Al, Ga,
and Ge, E including at least fifty atomic percent of the main
constituent, D including at least one constituent that includes Ir,
D including at least 50 atomic percent of the at least one
constituent, the templating structure being nonmagnetic at room
temperature; and a second magnetic layer on the templating
structure, at least one of the first magnetic layer and the second
magnetic layer including at least one of a Heusler compound and an
L1.sub.0 compound, the magnetic layer being in contact with the
templating structure and being magnetic as-deposited at room
temperature.
13. A method, comprising: providing a first magnetic layer;
providing a templating structure on the first magnetic layer, the
templating structure including D and E, a ratio of D to E being
represented by D.sub.1-xE.sub.x, with x being at least 0.4 and not
more than 0.6, E including a main constituent, the main constituent
including at least one of Al, is Ga, and Ge, E including at least
fifty atomic percent of the main constituent, D including at least
one constituent that includes Ir, D including at least 50 atomic
percent of the at least one constituent, the templating structure
being nonmagnetic at room temperature; and providing a second
magnetic layer on the templating structure, at least one of the
first magnetic layer and the second magnetic layer including at
least one of a Heusler compound and an L1.sub.0 compound.
14. The method of claim 13, wherein providing the templating
structure includes: depositing alternating layers of D and E.
15. The method of claim 13, wherein x is at least 0.47 and not more
than 0.54.
16. The method of claim 13, wherein D includes at least one of Ir
and IrRu.
17. The method of claim 13, wherein the at least one of the first
magnetic layer and the second magnetic layer includes at least one
of Mn.sub.3.1-yGe, Mn.sub.3.1-ySn, Mn.sub.3.1-zSb,
Mn.sub.3.1-sCo.sub.1.1-tSn, a MnGa alloy, a MnAl alloy, an FeAl
alloy, a MnGe alloy, a MnSb alloy, and a MnSn alloy, with y being
at least zero and not more than 0.6, z being at least 0 and not
more than 1.1, and with s being greater than zero and not more than
1.2 and t is greater than zero and not more than 1.0.
18. The method of claim 13, further comprising: providing an
additional templating structure underlying the first magnetic
layer, the additional templating structure including D' and E', an
additional ratio of D' to E' being represented by
D.sub.1-x'E.sub.x', with x' being at least 0.4 and not more than
0.6, E' including an additional main constituent, the additional
main constituent including at least one of Al, Ga, and Ge, E'
including at least fifty atomic percent of the main constituent, D'
including at least one additional constituent that includes at
least one of Ir, Co and Ru, D' including at least 50 atomic percent
of the at least one additional constituent, the additional
templating structure being nonmagnetic at room temperature.
19. The method of claim 13, further comprising: providing an
additional magnetic layer; and providing a tunneling barrier layer
between the additional magnetic layer the magnetic layer.
20. The method of claim 13, wherein at least one of providing the
first magnetic layer and providing the second magnetic layer
further includes: depositing the at least one of the first magnetic
layer and the second magnetic layer at room temperature.
Description
BACKGROUND OF THE INVENTION
[0001] Heusler compounds are a class of materials having the
representative formula X.sub.2YZ, where X and Y are transition
metals or lanthanides, and Z is from a main group element. Due to
the chemical distinction between X (or Y) and Z, they form a unique
structure defined by the space group symmetry L2.sub.1 (or
D0.sub.22 when they are tetragonally distorted), where four
face-centered cubic structures penetrate each other. The properties
of Heusler compounds are strongly dependent on the atomic ordering
of the elements constituting the compounds. Thus, the fabrication
of high quality Heusler films typically requires high temperature
thermal processes: for example, deposition at temperatures
significantly above room temperature and/or thermal annealing at
high temperatures (200.degree. C. or higher). Such high deposition
temperatures may adversely affect the properties of other portions
of the device in which the Heusler compound is desired to be used.
However, Heusler compounds and L1.sub.0 compounds have still
attracted interest as candidate materials for various spintronic
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] Various embodiments of the invention are disclosed in the
following detailed description and the accompanying drawings.
[0003] FIG. 1 illustrates an embodiment of the templating concept.
A and X represent transition metal elements, and E and Z represent
main group elements.
[0004] FIG. 2 illustrates an embodiment of the synthetic
antiferromagnetic (SAF) structure with a Heusler bilayer separated
by an IrAl spacer layer. X and X' represent transition metal
elements, and Z and Z' represent main group elements.
[0005] FIG. 3 shows XRD scans of 300 .ANG. thick Ir.sub.1-xAl.sub.x
films on a MgO/MgO(001) substrate. IrAl films were deposited at
room temperature and annealed in situ at various temperatures in
vacuum.
[0006] FIG. 4 shows a high resolution TEM image of 20 .ANG. thick
Mn.sub.3Sn film on an IrAl templating layer that was grown on a
MgO(001) single crystal substrate (not shown).
[0007] FIG. 5 shows root mean square (RMS) roughness (rims) versus
annealing temperature of an IrAl templating layer.
[0008] FIG. 6 presents room temperature perpendicular
magneto-optical Kerr effect (P-MOKE) hysteresis loops of 20 .ANG.
thick Mn.sub.3Sn films on a 300 .ANG. IrAl templating layer. The
IrAl layer was deposited at various target-to-substrate distances
with 135 mm being the typical value.
[0009] FIG. 7 shows a perpendicular MOKE hysteresis loop from an
IrAl templating layer grown on a MgO(001) single crystalline
substrate at room temperature.
[0010] FIG. 8 illustrates part of a device that incorporates the
templating and Heusler layers described herein.
[0011] FIG. 9 illustrates an embodiment of a magnetic tunnel
junction device that incorporates the templating and Heusler layers
described herein.
[0012] FIG. 10 shows the c-axis lattice spacing as a function of
IrAl and CoAl templating layer thicknesses.
[0013] FIG. 11 indicates the in-plane lattice constant for
different thicknesses of embodiments of IrAl and CoAl/IrAl
bilayers.
[0014] FIG. 12 shows P-MOKE hysteresis loops of a 30 .ANG. thick
Mn.sub.3Ge Heusler layer on a CoAl/IrAl templating bilayer that was
grown on a MgO(001) single crystalline substrate.
[0015] FIG. 13 shows P-MOKE hysteresis loops of a 20 .ANG. thick
Mn.sub.3Sn Heusler layer on the templating layers IrAl, CoAl/IrAl,
and CoAl, grown on a MgO(001) single crystalline substrate.
[0016] FIG. 14 illustrates layers formed when employing one method
herein (including two different multilayered structures), in which
a target in-plane lattice constant is selected prior to the
deposition of the layers.
[0017] FIG. 15 illustrates part of a device that incorporates
multilayered and Heusler layers described herein.
[0018] FIG. 16 illustrates an embodiment of a device that
incorporates a templating structure and a Heusler layer, which can
be used as a memory element in a racetrack device.
[0019] FIG. 17 illustrates an embodiment of a magnetic tunnel
junction device that incorporates a templating structure and a
Heusler layer, as described herein.
[0020] FIG. 18 illustrates an embodiment of a magnetic tunnel
junction device that incorporates a templating structure and a
Heusler layer that acts as a pinning layer.
[0021] FIG. 19 shows a P-MOKE hysteresis loop from the illustrated
Heusler stack, which includes an 11 .ANG. IrAl spacer layer. The
magnetic moments of the two Heusler-containing layers at different
positions in the hysteresis loop are indicated with thick arrows,
whereas the thin arrows indicate the scan direction. (The
illustrated Heusler stack on the left-hand side shows magnetic
moments corresponding to the case near zero field--compare with the
loop shown on the right-hand side.)
[0022] FIG. 20 shows P-MOKE hysteresis loops of embodiments of
Heusler stacks (e.g. left hand side of FIG. 20) having different
IrAl spacer layers of thicknesses 7, 9, and 11 .ANG..
[0023] FIG. 21 P-MOKE hysteresis loops of embodiments of Heusler
stacks (e.g. left hand side of FIG. 21) having different CoAl
spacer layer thicknesses, up to 16 .ANG..
[0024] FIG. 22 illustrates an embodiment of a synthetic
antiferromagnetic (SAF) structure that incorporates the templating
and Heusler layers described herein, in which the magnetic moments
of the Heusler layers are perpendicular to a multilayered structure
that separates the Heusler layers.
[0025] FIG. 23 illustrates an embodiment of a SAF that incorporates
the templating and Heusler layers described herein, in which the
magnetic moments of the Heusler layers are parallel to a
multilayered structure that separates the Heusler layers.
[0026] FIG. 24 illustrates an embodiment of a magnetic tunnel
junction device that incorporates the templating and Heusler layers
described herein, thereby forming a memory element that includes an
SAF.
DETAILED DESCRIPTION
[0027] The invention can be implemented in numerous ways, including
as a process; an apparatus; a system; a composition of matter; a
computer program product embodied on a computer readable storage
medium; and/or a processor, such as a processor configured to
execute instructions stored on and/or provided by a memory coupled
to the processor. In this specification, these implementations, or
any other form that the invention may take, may be referred to as
techniques. In general, the order of the steps of disclosed
processes may be altered within the scope of the invention. Unless
stated otherwise, a component such as a processor or a memory
described as being configured to perform a task may be implemented
as a general component that is temporarily configured to perform
the task at a given time or a specific component that is
manufactured to perform the task. As used herein, the term
`processor` refers to one or more devices, circuits, and/or
processing cores configured to process data, such as computer
program instructions.
[0028] A detailed description of one or more embodiments of the
invention is provided below along with accompanying figures that
illustrate the principles of the invention. The invention is
described in connection with such embodiments, but the invention is
not limited to any embodiment. The scope of the invention is
limited only by the claims and the invention encompasses numerous
alternatives, modifications and equivalents. Numerous specific
details are set forth in the following description in order to
provide a thorough understanding of the invention. These details
are provided for the purpose of example and the invention may be
practiced according to the claims without some or all of these
specific details. For the purpose of clarity, technical material
that is known in the technical fields related to the invention has
not been described in detail so that the invention is not
unnecessarily obscured.
[0029] New magnetic materials are needed to allow for scaling of
magnetic devices such as spin transfer torque magnetic random
access memories (STT MRAM beyond the 20 nm node. These materials
are desired to have very large perpendicular magnetic anisotropy
(PMA) and, for integration purposes, be compatible with
conventional CMOS technologies. Such magnetic materials form
electrodes of magnetic tunnel junction (MTJ) based memory elements.
A mechanism for switching the state of the MTJ element is using
spin polarized tunneling currents that are passed through the MTJ.
The magnitude of this current is limited by the size of the
transistors used to provide the write current. This means that the
thickness of the electrode must be sufficiently small that it can
be switched by the available current. For magnetization values of
1000 emu/cm.sup.3, the electrode should have a thickness that does
not exceed approximately 1 nm. Recently it has been shown that
using templating layers, such as CoAl, CoGa, CoSn, or CoGe, it is
possible to deposit an ultrathin Heusler layer (thickness of
.about.1 nm) having bulk-like magnetic properties. These ultrathin
Heusler compound films of even a single unit cell thickness showed
perpendicular magnetic anisotropy and square magnetic hysteresis
loops, making them candidate materials for use in STT-MRAM and
racetrack memory applications.
[0030] Moreover, the ultrathin Heusler compound films deposited on
templating layers, such as CoAl, CoGa, CoSn, or CoGe, grow
epitaxially, i.e., the ultrathin Heusler compound has the same or
substantially same in-plane lattice constant as that of the
templating layer. There exists a significant lattice mismatch
between these templating layers and the MgO tunnel barrier
(>5%), which may result in low tunnel magnetoresistance (TMR),
as a result of incoherent tunneling through the tunnel barrier.
Such a reduction in TMR is undesirable for device performance.
[0031] As mentioned above, the ultrathin Heusler compound films
with perpendicular magnetic anisotropy and square magnetic
hysteresis loops may be used in STT-MRAM and racetrack memory
applications. In such applications, a synthetic anti-ferromagnet
(SAF) may be used. In an STT-MRAM application, the reference layer
can include an SAF structure because this structure has very small
fringing fields, which are the primary cause of the offset fields
observed in the measured hysteresis loops of the storage layer. In
a racetrack memory, the domain wall velocities in nanowires of an
SAF structure are significantly higher than those in the nanowires
of conventional ferromagnets. The SAF structures formed from
conventional ferromagnets use Ru as the non-magnetic spacer layer.
The family of tetragonal Heusler compounds, which include Mn.sub.3Z
with Z.dbd.Ge, Sn, and Sb, have a layered structure of alternating
layers of Mn--Mn and Mn--Z. The use of a known elemental spacer
layer (e.g., Ru alone) does not work for structures that include
two Heusler layers, since elemental Ru is unable to replicate the
ordering of a Heusler layer underneath it; thus, it is unable to
promote the ordering in a Heusler layer grown over the Ru spacer
layer.
[0032] The exemplary embodiments are described in the context of
particular methods, magnetic junctions and magnetic memories having
certain components. One of ordinary skill in the art will readily
recognize that the present invention is consistent with the use of
magnetic junctions and magnetic memories having other and/or
additional components and/or other features not inconsistent with
the present invention. The method and system are also described in
the context of current understanding of the spin transfer
phenomenon, of magnetic anisotropy, and other physical phenomenon.
Consequently, one of ordinary skill in the art will readily
recognize that theoretical explanations of the behavior of the
method and system are made based upon this current understanding of
spin transfer, magnetic anisotropy and other physical phenomena.
However, the method and system described herein are not dependent
upon a particular physical explanation. One of ordinary skill in
the art will also readily recognize that the method and system are
described in the context of a structure having a particular
relationship to the substrate. One of ordinary skill in the art
will readily recognize that the method and system are consistent
with other structures. In addition, the method and system are
described in the context of certain layers being synthetic and/or
simple. However, one of ordinary skill in the art will readily
recognize that the layers could have another structure.
Furthermore, the method and system are described in the context of
magnetic junctions and/or substructures having particular layers.
One of ordinary skill in the art will readily recognize that
magnetic junctions and/or substructures having additional and/or
different layers not inconsistent with the method and system could
also be used. Moreover, certain components are described as being
magnetic, ferromagnetic, and ferrimagnetic. As used herein, the
term magnetic could include ferromagnetic, ferrimagnetic or like
structures. Thus, as used herein, the term "magnetic" or
"ferromagnetic" includes, but is not limited to ferromagnets and
ferrimagnets. As used herein, "in-plane" is substantially within or
parallel to the plane of one or more of the layers of a magnetic
junction. Conversely, "perpendicular" and "perpendicular-to-plane"
corresponds to a direction that is substantially perpendicular to
one or more of the layers of the magnetic junction. The method and
system are also described in the context of certain alloys. Unless
otherwise specified, if specific concentrations of the alloy are
not mentioned, any stoichiometry not inconsistent with the method
and system may be used.
[0033] The use of the terms "a" and "an" and "the" and similar
referents in the context of describing the invention (especially in
the context of the following claims) are to be construed to cover
both the singular and the plural, unless otherwise indicated herein
or clearly contradicted by context. The terms "comprising,"
"having," "including," and "containing" are to be construed as
open-ended terms (i.e., meaning "including, but not limited to,")
unless otherwise noted.
[0034] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. It is
noted that the use of any and all examples, or exemplary terms
provided herein is intended merely to better illuminate the
invention and is not a limitation on the scope of the invention
unless otherwise specified. Further, unless defined otherwise, all
terms defined in generally used dictionaries may not be overly
interpreted.
[0035] A device including a first magnetic layer, a templating
structure and a second magnetic layer is described. The templating
structure is on the first magnetic layer. The second magnetic layer
is on the templating structure. The templating structure includes D
and E. A ratio of D to E is represented by D.sub.1-xE.sub.x, with x
being at least 0.4 and not more than 0.6. E includes a main
constituent. The main constituent includes at least one of Al, Ga,
and Ge. E includes at least fifty atomic percent of the main
constituent. D includes at least one constituent that includes Ir.
D includes at least 50 atomic percent of the at least one
constituent. In some embodiments, D includes at least one of Ir and
IrRu. The templating structure is nonmagnetic at room temperature.
At least one of the first magnetic layer and the second magnetic
layer includes at least one of a Heusler compound and an LI.sub.0
compound. In some embodiments, the templating structure has a
thickness of at least three Angstroms and not more than eleven
Angstroms.
[0036] In some embodiments, the templating structure includes at
least one layer of D and at least one layer of E, the at least one
layer of E sharing an interface with the at least one layer of D.
In some embodiments, x is at least 0.47 and not more than 0.54. The
first and/or second magnetic layer may have a thickness of not more
than five nanometers.
[0037] In some embodiments, E includes at least one of Al, an AlGe
alloy, and an AlGa alloy. E may be selected from Al, AlSn, AlGe,
AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn. The at least one of the first
and the second magnetic layer includes at least one of
Mn.sub.3.1-yGe, Mn.sub.3.1-ySn, Mn.sub.3.1-zSb,
Mn.sub.3.1-xCo.sub.1.14Sn, a MnGa alloy, a MnAl alloy, an FeAl
alloy, a MnGe alloy, a MnSb alloy, and a MnSn alloy, with y being
at least zero and not more than 0.6, z being at least 0 and not
more than 1.1, and with s being greater than zero and not more than
1.2 and t is greater than zero and not more than 1.0.
[0038] In some embodiments, the device includes an additional
templating structure underlying the first magnetic layer. The
additional templating structure includes D' and E'. A ratio of D'
to E' is represented by D'.sub.1-x'E'x', where x' is at least 0.4
and not more than 0.6. E' includes an additional main constituent.
The additional main constituent includes at least one of Al, Ga,
and Ge. E' includes at least fifty atomic percent of the main
constituent. D' includes at least one additional constituent that
includes at least one of Ir, Co and Ru. D' includes at least 50
atomic percent of the at least one additional constituent, the
additional templating structure being nonmagnetic at room
temperature. The device may include an additional magnetic layer
and a tunneling barrier layer between the additional magnetic layer
and the magnetic layer.
[0039] A device including a plurality of memory elements is
described. Each of the memory elements includes a first magnetic
layer, a templating structure on the first magnetic layer and a
second magnetic layer on the first magnetic layer. The templating
structure includes D and E. A ratio of D to E is represented by
D.sub.1-xE.sub.x, with x being at least 0.47 and not more than
0.54. E includes a main constituent. The main constituent includes
at least one of Al, Ga, and Ge. E includes at least fifty atomic
percent of the main constituent. D includes at least one
constituent that includes Ir. D includes at least 50 atomic percent
of the constituent(s). The templating structure is nonmagnetic at
room temperature. At least one of the first magnetic layer and the
second magnetic layer includes at least one of a Heusler compound
and an L1.sub.0 compound. The magnetic layer is in contact with the
templating structure and is magnetic as-deposited at room
temperature.
[0040] A method for providing a device is also described. The
method includes providing a first magnetic layer, providing a
templating structure on the first magnetic layer, and providing a
second magnetic layer on the templating structure. The templating
structure includes D and E. A ratio of D to E is represented by
D.sub.1-xE.sub.x, with x being at least 0.4 and not more than 0.6.
In some embodiments, x is at least 0.47 and not more than 0.54. E
includes a main constituent. The main constituent includes at least
one of Al, Ga, and Ge. E includes at least fifty atomic percent of
the main constituent. D includes at least one constituent that
includes Ir. D includes at least 50 atomic percent of the
constituent(s). The templating structure is nonmagnetic at room
temperature. At least one of the first magnetic layer and the
second magnetic layer includes at least one of a Heusler compound
and an L1.sub.0 compound. The first magnetic layer and/or the
second magnetic layer may be deposited at room temperature.
[0041] In some embodiments, providing the templating structure
includes depositing alternating layers of D and E. In some
embodiments, x is at least 0.47 and not more than 0.54. D may
include or consist of at least one of Ir and IrRu. The first
magnetic layer and/or the second magnetic layer may include at
least one of Mn.sub.3.1-yGe, Mn.sub.3.1-ySn, Mn.sub.3.1-zSb,
Mn.sub.3.1-sCo.sub.1.1-tSn, a MnGa alloy, a MnAl alloy, an FeAl
alloy, a MnGe alloy, a MnSb alloy, and a MnSn alloy, with y being
at least zero and not more than 0.6, z being at least 0 and not
more than 1.1, and with s being greater than zero and not more than
1.2 and t is greater than zero and not more than 1.0.
[0042] In some embodiments, the method includes providing an
additional templating structure underlying the first magnetic
layer. The additional templating structure includes D' and E'. A
ratio of D' to E' is represented by D'.sub.1-x'E'.sub.x', with x'
being at least 0.4 and not more than 0.6. E' includes an additional
main constituent. The additional main constituent includes at least
one of Al, Ga, and Ge. E' includes at least fifty atomic percent of
the main constituent. D' includes at least one additional
constituent that includes at least one of Ir, Co and Ru. D'
includes at least 50 atomic percent of the at least one additional
constituent. The additional templating structure is nonmagnetic at
room temperature.
[0043] In some embodiments, the method includes providing an
additional magnetic layer and providing a tunneling barrier layer
between the additional magnetic layer and the magnetic layer.
[0044] IrAl templating layer(s) which promote growth of ultrathin
Heusler compound films with perpendicular magnetic anisotropy and
square magnetic hysteresis loops are described. Disclosed herein is
a templating layer whose in-plane lattice constant can be
pre-selected over a significant range that includes the lattice
constant of the MgO tunnel barrier. Disclosed herein is a spacer
layer that promotes the formation of an SAF structure between
Heusler layers. It is shown herein that an IrAl alloy spacer layer
having the CsCl structure induces anti-ferromagnetic coupling
between two tetragonal Heusler compound layers separated by that
spacer layer.
[0045] Disclosed herein are highly textured, very smooth, high
quality ultrathin films of Heusler compounds, which can be
fabricated without a thermal annealing process (or with a lower
temperature thermal annealing process), using a non-magnetic
templating layer. The templating layer may be formed from a binary
alloy of Ir--Al with the B2 structure, the cubic version of
L1.sub.0. The templating layer can be deposited at room temperature
and is atomically ordered (i.e., alternating atomic layers of Ir
and Al are formed), even in the as-deposited state. Ultrathin films
of Heusler compounds deposited on these templating layers are
highly epitaxial, atomically ordered, high quality films with
excellent magnetic properties, including especially high values of
perpendicular magnetic anisotropy and square magnetic hysteresis
loops (with the remanent moment in zero magnetic field being close
to the saturation moment). This is attributed to the similarity
between the B2 symmetry of the templating layer and the L2.sub.1 or
D0.sub.22 symmetry of the Heusler layer.
[0046] A characteristic of the underlayer (e.g. a templating layer)
is that it is composed of elements that are similar to those of the
Heusler compound. For example, any intermixing or diffusion of the
Al from the IrAl underlayers into the Heusler layer does not
significantly change the properties of the Heusler layer, since Al
is from the class of Z elements from which the Heuslers are formed.
Similarly, underlayers that partially replace Al with other Z
elements, such as Ga, Ge and/or Sn, are suitable for the
underlayers. The Ir within the IrAl underlayers can also diffuse
into the Heusler without causing significant degradation of the
magnetic properties of the Heusler layer. Thus, the underlayers are
advantageously formed from A-E alloys, where A is a transition
metal and E is a main group element.
[0047] Another property of the underlayer is that it can promote
the desired ordering of the Heusler compound. The underlayer will
generally have terraces with atomic steps between neighboring
terraces, in which each of the steps separates a terrace with a
surface formed from Ir from a terrace formed from Al. Due to the
chemical affinity of X (or Y) to Al, and of Z to Ir, the underlayer
promotes the desired ordering of the Heusler compound at modest
temperatures even as low as room temperature, as illustrated in
FIG. 1. Annealing does not significantly improve the magnetic
properties of the Heusler compound.
[0048] Such Heusler compounds are used herein to form memory
storage elements (e.g., racetrack and MRAM) and synthetic
anti-ferromagnets, which are disclosed herein. Associated methods
of formation, including a method of preselecting a desired lattice
constant of a structure are also disclosed.
[0049] IrAl Templating Layer
[0050] Single crystal epitaxial films of Ir.sub.1-xAl.sub.x alloy
were grown by dc-magnetron sputtering onto MgO buffer layers
overlying MgO (001) single crystal substrates, in an ultra-high
vacuum (UHV) chamber with a base pressure of
.about.1.times.10.sup.-9 Torr. The MgO ubstrates were cleaned in an
ultrasonic bath of methanol for 30 min, treated in an isopropyl
alcohol (IPA) vapor degreaser for 2 min, dried with N.sub.2 gas at
65.degree. C. for 15 min, and then transferred into the deposition
chamber where they were annealed at 650.degree. C. for 30 min in
ultra-high vacuum (UHV). The MgO buffer layer was prepared by
depositing 20 .ANG. thick MgO at room temperature using
rf-magnetron sputtering from a MgO target. For all the magnetron
sputtering processes, Argon was the sputtering gas at a typical gas
pressure of 3 mTorr. Films of 300 .ANG. thick IrAl were deposited
at room temperature. These films were either not annealed or
annealed at various temperatures T.sub.AN=200, 300, 400, and
500.degree. C. for 30 minutes. The composition of the IrAl layers
was determined to be Ir.sub.51.6Al.sub.48.4 by Rutherford
backscattering (RBS) measurement. Elements other than those
discussed herein may be present in the disclosed structures. For
example, although pristine Ir and Al layers are frequently used,
these layers may include other elements that constitute a
significant atomic percentage of these layers (e.g., up to 50
atomic percent).
[0051] X-ray diffraction (XRD) .theta.-2.theta. scans were
performed on these films. These XRD measurements were performed
using a Bruker D8 Discover system at room temperature. FIG. 3 shows
XRD scans of IrAl films annealed at various temperatures T.sub.AN
for 30 minutes. These IrAl films were deposited from a single IrAl
alloy target. The data were compared with those taken from an IrAl
film which was not annealed. The data show the main IrAl (002) peak
at 2.theta.=.about.57.degree. as well as the IrAl (001) peak at
2.theta.=.about.27.5.degree.. The existence of the IrAl (001)
superlattice peak clearly indicates that there is an alternate
layering of Ir and Al even in the absence of annealing; even when
annealing is employed, the alternate layering structure is
preserved. The X-ray diffraction associated with the substrate was
observed for all samples and is labeled as the MgO(002) peak.
[0052] FIG. 4 is a high resolution, scanning transmission electron
microscopy (HR-STEM) image of a typical 20 .ANG. Mn.sub.3Sn layer
grown on an IrAl templating layer. The stack of this sample was
MgO(001)/20 .ANG. MgO/300 .ANG. IrAl/20 .ANG. Mn.sub.3Sn/20 .ANG.
MgO/20 .ANG. Ta. The IrAl templating layer consists of alternating
layers of Ir and Al in agreement with the XRD measurements detailed
in FIG. 3. The epitaxial matching of 20 .ANG. thick Mn.sub.3Sn
layers with the IrAl templating layer is clearly seen. Furthermore,
the desired ordering within the Heusler layer is evident: The
Mn--Sn layers are also distinguishable from the Mn--Mn layers,
showing the alternating atomic layers and even the desired ordering
within each Mn--Sn layer.
[0053] Atomic force microscopy was performed to probe the surface
morphology of 300 .ANG. thick IrAl templating layers. FIG. 5 shows
the root-mean-squared surface roughness (rims) for various
annealing temperatures. The films show a very smooth surface
independent of annealing temperature, with r.sub.rms.about.1
.ANG..
[0054] Mn.sub.3Z Tetragonal Heusler
[0055] 20 .ANG.-thick Mn.sub.3Sn films were deposited at room
temperature by magnetron sputter deposition on an IrAl templating
layer. The stacks were capped by 20 .ANG. thick MgO and 20 .ANG.
thick Ta to prevent ambient oxidation of the Heusler layer. The
resulting structures were of the form: MgO(001)/20 .ANG. MgO/300
.ANG. IrAl/20 .ANG. Mn.sub.3Sn/20 .ANG. MgO/20 .ANG. Ta. The IrAl
layers were deposited at room temperature with various
substrate-to-target distances. The typical substrate-to-target
distance is .about.135 mm in the deposition tool. Four deposition
positions were evaluated: 135 mm, 125 mm, 120 mm, and 113 mm (i.e.,
the substrate was also placed closer to the target than the typical
substrate-to-target distances of 135 mm, by 10, 15, and 22 mm,
respectively). Table 1 (see end of specification) includes the RBS
composition of the IrAl layer at these four deposition positions.
As the substrate-to-target distance is decreased, the Ir content of
the film is decreased. FIG. 6 shows the perpendicular
magneto-optical Kerr effect (P-MOKE) hysteresis loops of Mn.sub.3Sn
layer for deposition positions of 135 mm, 125 mm, and 120 mm.
Interestingly, the magnetic properties of Mn.sub.3Sn are
independent of these deposition positions, showing excellent PMA
for all the films using an IrAl templating layer. Abrupt switching
of the magnetic moment implies that there is no second phase. These
results clearly demonstrate that an IrAl layer at compositions
close to 1:1 promotes ordering within an ultrathin Mn.sub.3Sn
Heusler compound, even when deposited at room temperature without
any subsequent annealing, with the Heusler compound showing
excellent PMA. FIG. 7 shows the P-MOKE signal from a MgO(001)/20
.ANG. MgO/300 .ANG. IrAl/20 .ANG. MgO/20 .ANG. Ta sample, which is
the sample without the Heusler layer. The lack of any magnetic
field-dependent MOKE signal clearly indicates that the IrAl
templating layer is non-magnetic at room temperature. Though the
data discussed above were collected with a structure that includes
an IrAl templating layer and a Mn.sub.3Sn Heusler compound, the
IrAl templating layer is equally effective in promoting desired
ordering when used with Mn.sub.3Ge, Mn.sub.3Sb and Mn.sub.3Ga
Heusler compounds. The Heusler compounds can be Mn.sub.3.1-xGe,
Mn.sub.3.1-xSn, Mn.sub.3.1-xSb, and Mn.sub.3.1-xGa, with x being in
the range from 0 to 1.1. Alternatively, the Heusler compound used
may be a ternary Heusler, such as Mn.sub.3.1-xCo.sub.1.1-ySn,
wherein x.ltoreq.1.2 and y.ltoreq.1.0. Furthermore, the IrAl
templating layer is effective in inducing order within L1.sub.0
compounds, whose constituent elements include one transition metal
element and a main group element. Possible L1.sub.0 compounds
include MnAl alloys, MnGa alloys, MnSn alloys, MnGe alloys, MnSb
alloys, and FeAl alloys.
[0056] The structural ordering of ultrathin layers is likely due to
the differing chemical properties of the elements Ir and Al in the
templating layer. As an alternative to Al, Al alloys such as AlSn,
AlGe, AlGaGe, AlGaSn, AlGeSn, and AlGaGeSn may be employed.
Furthermore, using an Ir--Co alloy instead of just Ir within the
IrAl templating layer (with the Ir--Co alloy having the composition
Ir.sub.xCo.sub.1-x, with x being in the range 0.0001 to 0.9999)
will also promote structural ordering of ultrathin Heusler layers.
Binary (X=Y) and ternary Heusler alloys consist of two or three
different types of atoms, respectively. In X.sub.2YZ Heuslers, the
Z main group element typically has a high chemical affinity for X
and Y. In this context, the formation of an ordered structure
should take place, irrespective of the choice of Z.
[0057] The structure described above can be used in racetrack
memory devices. For example, the racetrack may be a nanowire that
may include a substrate, an optional seed layer, a templating
layer, and a first magnetic layer of a Heusler compound (see FIG.
8). (The electrode on the left side of the structure is part of the
electrical circuit used to pass flow current through the
device--see also FIGS. 9, 14-18, and 22-24. The electrodes called
out in the various stacks herein represent magnetic layers.)
Magnetic domain walls may be moved along this racetrack, as
described in U.S. Pat. No. 6,834,005. Data may be read out of (and
stored in) the racetrack by interrogating (or changing) the
orientation of the magnetic moment of the magnetic material between
adjacent domain walls within the racetrack.
[0058] FIG. 8 illustrates a device that includes an optional seed
layer overlying a substrate. The top two layers form a device that
includes a magnetic (e.g. Heusler) layer that overlies a templating
layer (multi-layered structure) of the sort described herein. This
multi-layered structure is non-magnetic at room temperature and
includes alternating layers of D and E, which are now defined. (The
multi-layered structure of FIG. 8 is shown in greater detail in
FIG. 1, where D is represented by the letter A.) E comprises at
least one of (e.g. one, two or more of) Al, Ga, and Ge, with the
composition of the structure being represented by D.sub.1-xE.sub.x,
in which x is in the range from 0.47 to 0.54 in some embodiments
(e.g. at least 0.4 and not more than 0.6 in some embodiments), and
with this main group element representing at least 50 atomic
percent of E. On the other hand, D comprises at least one element
that includes Ir, with Ir representing at least 50 atomic percent
of D. (D may optionally include up to at least 10 percent or more
of Co.) The magnetic layer may advantageously be a pure Heusler
compound; alternatively, it may include a Heusler and/or an
L1.sub.0 compound (e.g., a compound selected from the group
consisting of MnGa, MnAl, FeAl, MnGe, MnSb, and MnSn alloys). This
magnetic layer (which may be less than 5 nm thick; data obtained
with a thickness of just one unit cell show satisfactory magnetism)
may contact and overlie the multi-layered structure, as shown in
FIG. 8. Other thicknesses and compositions are possible.
[0059] In some embodiments, the Heusler compound of FIG. 8 may be
selected from the group consisting of Mn.sub.3.1-xGe,
Mn.sub.3.1-xSn, and Mn.sub.3.1-xSb, with x being in the range from
0 to not more than 1.1. Alternatively, the Heusler compound may be
a ternary Heusler, e.g., Mn.sub.3.1-xCo.sub.1.1-ySn, in which
x.ltoreq.1.2 and y.ltoreq.1.0. The Heusler may also advantageously
include Co.
[0060] Standard deposition techniques may be used to form the
devices of FIG. 8 and FIG. 9 (discussed below). By way of example,
Ir and Al may be deposited onto a substrate, thereby forming a
composite layer on it. The composite layer may then be annealed (at
a temperature of at least 200.degree. C. and at least 400.degree.
C. in some embodiments), so that at least one layer of Ir and at
least one layer of Al are formed from the composite layer,
resulting in the formation of the multi-layered structure.
Additional layers, such as a magnetic (Heusler) layer may be formed
over the multi-layered structure (e.g., via deposition).
[0061] The structure described above in connection with FIG. 8 can
also be used as part of an MRAM memory device, and one such MRAM
memory element is shown in FIG. 9. As with MRAM elements generally,
a tunnel barrier is situated between two magnetic electrodes, one
of which has a fixed magnetic moment and the other of which has a
magnetic moment that is switchable, thereby permitting the
recording and erasing of data. The Heusler layer, which may be
either ferro- or ferri-magnetic, overlies a templating layer which
in turn overlies a substrate; the Heusler layer has its magnetic
moment aligned perpendicular to the layer plane. An optional seed
layer may be interposed between the substrate and the templating
layer. An optional polarization enhancement layer may be used to
increase performance and may include Fe, a CoFe alloy, and/or
Co.sub.2MnSi. The tunnel barrier may be MgO (001), although other
(001)-oriented tunnel barriers may be used, such as CaO and LiF. In
some embodiments, MgAl.sub.2O.sub.4 can be used as a tunnel
barrier, with its lattice spacing being selected by controlling the
Mg--Al composition, thereby resulting in better lattice matching
with the Heusler compounds (e.g., the composition of this tunnel
barrier can be represented as Mg.sub.1-zAl.sub.2+(2/3)zO.sub.4,
wherein -0.5<z<0.5). The magnetic electrode overlying the
tunnel barrier may comprise Fe, a CoFe alloy, or a CoFeB alloy, for
example. If the magnetic layer on top of the barrier has a fixed
magnetic moment, then its moment can be stabilized by placing a
synthetic antiferromagnet (SAF) in proximity with it (e.g., above
it, as shown). The capping layer may comprise Mo, W, Ta, Ru, or a
combination thereof. Current may be induced by applying a voltage
between the two magnetic electrodes, which are separated by the
tunnel barrier.
[0062] IrAl Templating Layer with Tunable Lattice Constant
[0063] FIG. 10 shows the out-of-plane c-axis lattice spacing of the
Ir--Al and Co--Al templating layers. The resulting structures are
of the form MgO(001)/20 .ANG. MgO/x .ANG. IrAl/20 .ANG. MgO/20
.ANG. Ta where x=30, 45, 60, 75, 100, 200, 300 and 500 .ANG. and
MgO(001)/20 .ANG. MgO/y .ANG. CoAl/20 .ANG. MgO/20 .ANG. Ta, where
y=50, 100, 150, and 300 .ANG.. The c-axis lattice spacing was
determined by X-ray diffraction (XRD) 0-20 scans. For the
embodiments shown, the c-axis lattice spacing for the CoAl
templating layer is independent of the CoAl thickness studied here,
whereas the c-axis lattice spacing for the IrAl templating layer
varies systematically from 3.05 to 3.25 .ANG., as a function of the
IrAl layer thickness. This variation can be extended even further
by using a bilayer of CoAl/IrAl. Assuming the validity of the
Poisson effect (i.e., unit cell volume is conserved) and using the
known value for the volume of the IrAl unit cell, the in-plane
lattice constant for IrAl templating layers can be estimated. FIG.
11 is a plot of the calculated in-plane lattice constant for
various thicknesses of IrAl in a single IrAl layer or within an
IrAl/CoAl bilayer. (Note that the possibly limiting cases of CoAl
only and MgO are also included for comparison.) In some
embodiments, the in-plane lattice constant can be selected in
advance to be anywhere from .about.2.83 to .about.3.05 A (e.g. at
least 2.8 Angstroms and not more than 3.1 Angstroms in some
embodiments). This range includes (overlaps with) the lattice match
to the MgO tunnel barrier.
[0064] Mn3Z Heusler Compound
[0065] An IrAl templating layer is disclosed that is capable of
inducing order in ultra-thin Mn.sub.3Z (e.g. Mn.sub.3Ge,
Mn.sub.3Sn, and Mn.sub.3Sb) Heusler films including when deposited
at room temperature. The stack discussed herein consists of
MgO(001)/20 .ANG. MgO/100 .ANG. CoAl/300 .ANG. IrAl/30 .ANG.
Mn.sub.3Ge/20 .ANG. MgO/20 .ANG. Ta. Thus, Mn.sub.3Ge is discussed.
FIG. 12 shows the P-MOKE hysteresis loops obtained for this
structure. A square hysteresis loop indicates substantial remanent
magnetization at zero applied field. This result demonstrates that
the IrAl templating layer can induce order in an ultrathin
Mn.sub.3Ge layer even after deposition of the Heusler compound at
room temperature. Likewise, an IrAl templating layer is effective
in promoting order within Mn.sub.3Sb and Mn.sub.3Ga Heusler
compounds. The binary Heusler compounds may advantageously have a
composition given by Mn.sub.3.1-xGe, Mn.sub.3.1-xSn,
Mn.sub.3.1-xSb, and Mn.sub.3.1-xGa, with x being in the range from
0 to not more than 1.1. Alternatively, the Heusler compound may be
a ternary Heusler, such as Mn.sub.3.1-xCo.sub.1.1-ySn, wherein
x.ltoreq.1.2 and y.ltoreq.1.0. As for the CoAl templating layer, an
IrAl templating layer may be effective in inducing order within
L1.sub.0 compounds, whose constituent elements include one
transition metal element and a main group element. Possible
L1.sub.0 compounds include MnAl alloys, MnGa alloys, MnSn alloys,
MnGe alloys, MnSb alloys, and FeAl alloys.
[0066] FIG. 13 includes P-MOKE square hysteresis loops for 20 .ANG.
Mn.sub.3Sn Heusler layers deposited on IrAl, CoAl/IrAl, and CoAl
templating layers. This illustrates that the embodiment of
Mn.sub.3Sn Heusler layer is PMA even when its in-plane lattice
constant is varied from .about.2.84 to 2.92 .ANG. (e.g. 2.8
Angstroms to 2.95 Angstroms in some embodiments).
[0067] Based on the results described above, a target in-plane
lattice constant can be achieved with either of the configurations
of the templating structure shown in FIGS. 14 and 15. A first
configuration of the templating structure (see FIG. 14) may include
Ir and Co-containing multilayered structures in contact with each
other, in either order. The thicknesses of the Ir and Co-containing
multilayered structures are chosen to achieve the target in-plane
lattice constant. A second configuration of the templating
structure (see FIG. 15) may employ various thicknesses of the
Ir-containing multilayered structure or a Co-containing
multilayered structure.
[0068] Thus, the technology described herein lends itself to a
method of forming a device whose associated lattice constant can be
engineered. For example, for a device that includes at least one
multi-layered structure and a first magnetic layer (e.g. a Heusler
and/or an L1.sub.0 compound, having at thickness of less than 5 nm)
grown over (e.g., in contact with) the multi-layered structure,
such a method includes selecting a target lattice constant for the
first magnetic layer. If the first magnetic layer is a Heusler
compound, it may be selected from the group consisting of
Mn.sub.3.1-xGe, Mn.sub.3.1-xSn, and Mn.sub.3.1-xSb, with x being in
the range from at least 0 to not more than 1.1. The magnetic layer
may be also be advantageously doped with Co. Alternatively, the
Heusler compound may be a ternary Heusler of the form
Mn.sub.3.1-xCo.sub.1.1-ySn, wherein x.ltoreq.1.2 and y.ltoreq.1.0.
If the first magnetic layer is an L1.sub.0 compound, it may be
selected from the group consisting of MnGa, MnAl, FeAl, MnGe, MnSb,
and MnSn alloys.
[0069] A multi-layered Ir-containing structure (that is
non-magnetic at room temperature) is grown, in which the
Ir-containing structure has a lattice constant and comprises
alternating layers of Ir with E. The composition of the
Ir-containing structure is represented by Ir.sub.1-xE.sub.x,
wherein E comprises at least one other element that includes Al,
with x being in the range from 0.4 to not more than 0.6 (e.g. at
least 0.47 to not more than 0.54). The Ir-containing structure is
grown such that its lattice constant matches the target lattice
constant, by choosing the number of Ir layers and the number of Al
layers in the Ir-containing structure, so that the desired
thickness of the Ir-containing structure is obtained. In some
embodiments, "matching" includes other than exact matches. For
example, "matching" may include the Ir-containing structure's
lattice constant being within five percent of the target lattice
constant. In some embodiments, "matching" includes the
Ir-containing structure's lattice constant being not more than
three percent different from the target lattice constant. In some
such embodiments, "matching" includes the Ir-containing structure's
lattice constant being not more than one percent different from the
target lattice constant. As indicated in FIG. 11, this thickness
(for a given structure) determines the associated lattice constant.
A magnetic layer is grown over the Ir-containing structure (either
in contact with it, or in proximity with it as when an additional
multi-layered structure is incorporated into the stack). In some
embodiments, the thickness of the structure is at least twenty
Angstroms and not more than six hundred Angstroms. In some such
embodiments, thickness may be at least thirty Angstroms. In some
embodiments, the templating structure has a thickness of not more
than five hundred and fifty Angstroms. In some such embodiments,
the thickness of the templating structure is not more than five
hundred and ten Angstroms. In some embodiments, the templating
structure has a thickness of not more than five hundred
Angstroms.
[0070] In some embodiments, prior to growing the Ir-containing
structure, a multi-layered Co-containing structure is grown that is
non-magnetic at room temperature, with the Co-containing structure
having a lattice constant and including alternating layers of Co
with E'. The composition of the Co-containing structure is
represented by Co.sub.1-yE'.sub.y, in which E' comprises at least
one other element that includes Al, with y being in the range of
0.4 to not more than 0.6 (e.g. at least 0.47 to not more than
0.54). In some embodiments, E and/or E' may be an AlGe alloy.
Similarly, in some embodiments, E and/or E' may be an AlGa
alloy.
[0071] The Co-containing structure is grown by choosing the number
of Co layers and the number of Al layers in the Co-containing
structure, so that the desired thickness of the Co-containing
structure is obtained for the target lattice constant (e.g. as
indicated in FIG. 11). In some embodiments, the Co-containing
structure is in contact with the Ir-containing structure. For
example, the Ir-containing structure may overlie the Co-containing
structure or the Co-containing structure may be sandwiched between
the Ir-containing structure and the first magnetic layer. In some
embodiments, the Ir-containing structure and the Co-containing
structure together form a templating structure having a thickness
in the range of at least ten Angstroms to not more than six hundred
Angstroms (e.g. at least 20 to not more than 500 .ANG. in some
cases), with an in-plane lattice constant in the range of 2.7
Angstroms to not more than 3.1 Angstroms (e.g. at least 2.82 .ANG.
to not more than 3.03 .ANG.). This templating structure may be
advantageously annealed prior to depositing the first magnetic
layer. In some embodiments, the anneal temperature is at least
200.degree. C.
[0072] In the event that the structure is extended to include a
tunnel barrier (see below in connection with FIGS. 17 and 18), the
method includes forming a tunnel barrier over the first magnetic
layer. In operation, current may passes through both the tunnel
barrier and the first magnetic layer. An optional polarization
enhancement layer between the first magnetic layer and the tunnel
barrier may also be included. A second magnetic layer above (e.g.,
in contact with) the tunnel barrier may also be included. The
tunnel barrier may be MgO or Mg.sub.1-zAl.sub.2+(2/3)xO.sub.4,
wherein -0.5<z<0.5. As shown in the embodiments of FIGS. 17
and 18, a capping layer may be deposited over (e.g., in contact
with) the second magnetic layer. In one aspect of the method, the
structure may be formed so that current can pass through the tunnel
barrier and both magnetic layers.
[0073] A structure including or consisting of a templating
structure with a Heusler layer placed on a substrate with an
optional seed layer can be used to fabricate racetrack memory
devices. An embodiment of such a structure is depicted in FIG. 16.
Magnetic domain walls may be moved along such a racetrack. Data may
be read out of (and stored in) the racetrack by interrogating (or
changing) the orientation of the magnetic moment of the magnetic
material between adjacent domain walls within the racetrack.
[0074] The structure described above with respect to FIG. 16 can
also be used as part of a MRAM memory device. Two possible
configurations are shown in FIGS. 17 and 18, in which the
switchable layers are different, with the Heusler layer described
herein being switchable in FIG. 17 but not in FIG. 18. As with MRAM
elements generally, a tunnel barrier is situated between two
magnetic electrodes, one of which has a fixed magnetic moment and
the other of which has a magnetic moment that is switchable,
thereby permitting the recording and erasing of data. In some
embodiments, the Heusler layer, which may be either ferro- or
ferri-magnetic, overlies a templating layer which in turn overlies
a substrate; the Heusler layer has its magnetic moment aligned
perpendicular to the layer plane. An optional seed layer may be
interposed between the substrate and the templating layer. An
optional polarization enhancement layer may be used to increase
performance and may include Fe, a CoFe alloy, and/or Co.sub.2MnSi.
The tunnel barrier may be MgO(001), although other (001)-oriented
tunnel barriers may be used, such as CaO and LiF. In some
embodiments, MgAl.sub.2O.sub.4 can be used as a tunnel barrier
whose lattice spacing can be tuned (engineered) by controlling the
Mg--Al composition to result in better lattice matching with the
Heusler compounds (e.g., the composition of this tunnel barrier can
be represented as Mg.sub.1-zAl.sub.2+(2/3)zO.sub.4, wherein
-0.5<z<0.5). The magnetic electrode overlying the tunnel
barrier may comprise Fe, a CoFe alloy, and/or a CoFeB alloy, for
example. If the magnetic layer on top of the barrier has a fixed
magnetic moment, its moment can be stabilized by placing a
synthetic antiferromagnet (SAF) in proximity with it. The capping
layer may comprise Mo, W, Ta, Ru, or a combination thereof. Current
may be induced by applying a voltage between the two magnetic
electrodes, which are separated by the tunnel barrier.
[0075] IrAl Templating Layer as a SAF Spacer Layer
[0076] FIG. 2 shows that the use of an IrAl spacer layer between
layers of Heusler compounds can promote the formation of an SAF
structure. The IrAl alloy spacer layer forms a structure of the
CsCl structure type (Ir and Al form alternating layers) on top of
the bottom Heusler layer (X.sub.3Z in FIG. 2). This IrAl alloy
spacer layer templates the top Heusler layer (X'.sub.3Z' in FIG. 2)
and promotes anti-ferromagnetic coupling between two tetragonal
Heusler layers. IrGa, an alloy which also has the CsCl structure,
is expected to induce similar Heusler SAF structures, in view of
theoretical predictions. Moreover, substituting Ir-Ru for Ir within
IrAl (Ir.sub.xRu.sub.1-x with x in the range 0.0001 to 0.9999) will
also induce anti-ferromagnetic coupling between two tetragonal
Heusler compound layers. Likewise, IrGe would be expected to work
as a templating layer for the top Heusler layer. Alternatively, an
alloy of two or more of Al, Ga, and Ge could be used in combination
with Ir or Ir--Ru.
[0077] FIG. 19 includes a P-MOKE hysteresis loop obtained from a
sample with two different layers of Heusler compounds separated by
a non-magnetic spacer layer of IrAl. The stack of this sample was
MgO(001)/20 .ANG. MgO/100 .ANG. CoAl/12 .ANG. Mn.sub.3Sn/t=11 .ANG.
IrAl/20 .ANG. Mn.sub.2.4Sb/20 .ANG. MgO/20 .ANG. Ta (where "t"
represents thickness). Three distinct hysteresis loops are
observed, and the sets of pairs of arrows overlaid in FIG. 19
indicate the orientations of the magnetization of the Mn.sub.3Sn
and Mn.sub.2.4Sb layers. At high applied fields (e.g. greater than
1.5 kOe), the magnetizations of the two Heusler compounds are
parallel to each other and align with the externally applied field.
At zero applied field, in the remanent state, the magnetizations of
the two Heusler compounds are anti-parallel to each other. Thus,
the presence of the 11 .ANG. IrAl spacer layer separating the two
Heusler compounds promotes the formation of a synthetic
anti-ferromagnet (SAF). Moreover, the Heusler compounds, along with
their corresponding spacer layer, were deposited at room
temperature, with the resulting SAF structure needing no subsequent
annealing. Furthermore, in the case of two layers of Heusler
compounds having magnetic moments that are in-plane, the presence
of an 11 .ANG. IrAl spacer layer separating these two Heusler
layers also leads to the formation of a synthetic anti-ferromagnet
(SAF). More explicitly, here the magnetic moments of the Heusler
layers are substantially parallel to the interfaces between the
Heusler layers and the IrAl spacer layer separating them. In some
embodiments, the magnetic moments of the two Heusler layers may be
substantially anti-parallel to each other when the IrAl spacer
layer has a thickness in the range of at least 3 to not more than
11 .ANG.. In some embodiments, the IrAl spacer has a thickness of
at least eight and not more than ten Angstroms. Moreover, the
spacer layer providing anti-ferromagnetic coupling between the two
Heusler compounds can be of the form D'Al (where D' represents Ir
or an IrRu alloy) or IrE' (where E' is selected from the group
consisting of Al, Ga, and Ge, and combinations thereof) or D'E'
(where, for example, D' is an IrRu alloy and E' is an AlGa
alloy).
[0078] Although the thickness of each of the Heusler layers within
the SAF structure used herein was 1-2 nm, it is possible to form
SAF structures with other thicknesses (e.g. significantly thicker)
of Heusler layers. The Heusler layers within the bilayer may have
thicknesses of less than 5 nm, or even less than 3 nm, or as little
as the thickness of a single unit cell (e.g., 0.7-0.8 nm). Though
formation of the SAF is demonstrated herein for two Heusler
compounds, in general the Heusler compounds can be selected from
the group including or consisting of Mn.sub.3.1-xGe,
Mn.sub.3.1-xSn, and Mn.sub.3.1-xSb, with x being in the range from
0 to 1.1. Alternatively, the Heusler compounds may be a ternary
Heusler, such as Mn.sub.3.1-xCo.sub.1.1-ySn, wherein x.ltoreq.1.2
and y.ltoreq.1.0. The Heusler SAF structure can comprise a ternary
Heusler compound as either the first Heusler layer, the second
Heusler layer, or both Heusler layers.
[0079] Mn.sub.2.3-2.4Sb may be considered a Heusler or as part of
the family of L1.sub.0 compounds. Hence the results discussed above
indicate that the IrAl templating spacer layer would also be
effective in inducing SAF ordering between two L1.sub.0 compounds
(whose constituent elements include one transition metal element
and a main group element). Possible L1.sub.0 compounds include MnAl
alloys, MnGa alloys, MnSn alloys, MnGe alloys, and FeAl alloys.
[0080] FIG. 20 summarizes the P-MOKE hysteresis loops measured from
samples having two layers of different Heusler compounds separated
by a non-magnetic spacer layer of IrAl (of varying thickness, t).
The stack of these samples was MgO(001)/20 .ANG. MgO/100 .ANG.
CoAl/12 .ANG. Mn.sub.3Sn/t=7, 9, and 11 .ANG. IrAl/20 .ANG.
Mn.sub.2.4Sb/20 .ANG. MgO/100 .ANG. Ta. For all these samples with
t=7, 9, and 11 .ANG. of IrAl, the coupling between the Heusler
layers is anti-ferromagnetic.
[0081] FIG. 21 compares the P-MOKE hysteresis loops measured from
samples with two Heusler compound layers separated by a
non-magnetic spacer layer of CoAl. The spacer layer thickness t is
varied from 0 to 16 .ANG. (t=0, 4, 6, 7, 9, 10, 12, 14 and 16
.ANG.). The stack of these samples was MgO(001)/20 .ANG. MgO/50
.ANG. CoAl/12 .ANG. Mn.sub.3Ge/t CoAl/20 .ANG. Mn.sub.2.3Sb /20
.ANG. MgO/20 .ANG. Ta. The hysteresis loops obtained for samples
with a CoAl spacer layer show a single square hysteresis loop at
all CoAl thicknesses studied, which is different from the
hysteresis loops obtained for samples with an IrAl spacer layer.
The Heusler compound layers separated by the CoAl spacer layer are
coupled ferromagnetically for all thicknesses, and there is no
evidence that an SAF structure is formed.
[0082] A structure comprising a templating layer and a Heusler SAF,
grown on a substrate with an optional seed layer, can be used to
fabricate racetrack memory devices (e.g. the devices depicted in
FIGS. 22 and 23). Magnetic domain walls may be moved along such a
racetrack. Data may be read out of (and stored in) the racetrack by
interrogating (or changing) the orientation of the magnetic moment
of the magnetic material between adjacent domain walls within the
racetrack.
[0083] FIG. 22 shows a synthetic antiferromagnet that employs
magnetic Heusler layers, such as those described herein. The top
three layers of the structure shown in FIG. 22 are now described in
turn. The bottom-most of these three structures is a first magnetic
layer that includes a Heusler compound (and/or an L1.sub.0
compound, such as one or more of MnGa, MnAl, FeAl, MnGe, MnSb, and
MnSn).
[0084] Overlying the first magnetic layer is a first multi-layered
structure that is non-magnetic at room temperature; the first
multi-layered structure (i) overlies the first magnetic layer and
(ii) includes alternating layers of D' and E', in which E'
comprises a member selected from a first group consisting of Al,
Ga, Ge, and combinations thereof. The composition of the first
multi-layered structure can be represented by D'.sub.1-yE'.sub.y,
with y being in the range from at least 0.4 to not more than 0.
(e.g. at least 0.47 to not more than 0.54), and the selected member
of the first group representing at least 50 atomic percent of E'.
On the other hand, D' comprises a member selected from a second
group consisting of Ir and an IrRu alloy, in which the selected
member of the second group represents at least 50 atomic percent of
D'.
[0085] Overlying the first multi-layered structure is a second
magnetic layer that includes a Heusler compound (and/or an L1.sub.0
compound, such as those described above in connection with the
first magnetic layer). The second magnetic layer is in contact with
and overlies the first multi-layered structure. Together, the first
magnetic layer, the first multi-layered structure, and the second
magnetic layer (the top three layers shown in FIG. 22) form a
synthetic antiferromagnet (SAF).
[0086] In some embodiments, a second multi-layered structure
underlies and is contact with the SAF (e.g. the templating layer of
FIG. 22). This second multi-layered structure acts as a templating
layer for the SAF. It is non-magnetic at room temperature and
includes alternating layers of D and E, in which E comprises a
member selected from a third group consisting of Al, Ga, Ge, and
combinations thereof. The composition of the second multi-layered
structure is represented by D.sub.1-xE.sub.x, with x being in the
range from 0.4 to not more than 0.6 (e.g. 0.47 to 0.54), and the
selected member of the third group represents at least 50 atomic
percent of E (i.e. E includes at least fifty atomic percent of Al,
Ga, Ge, or a combination thereof). D comprises a member selected
from a fourth group consisting of Ir, Co, Ru, and combinations
thereof, wherein the selected member of the fourth group represents
at least 50 atomic percent of D (i.e. D includes at least fifty
atomic percent of Ir, Co, Ru, or a combination thereof). This
second multi-layered structure underlies and contacts the first
magnetic layer.
[0087] In the device of FIG. 22, the magnetic moments of the first
and second magnetic layers are substantially perpendicular to the
interfaces between (i) the first multi-layered structure and (ii)
the first and second magnetic layers, respectively. In FIG. 23, a
similar device is shown in which the magnetic moments of the first
and second magnetic layers are substantially parallel to the
interfaces between (i) the first multi-layered structure and (ii)
the first and second magnetic layers, respectively. The devices of
FIGS. 22 and 23 are otherwise identical in some embodiments. Note
that as with SAF structures generally, the magnetic moments of the
first and second magnetic layers are substantially anti-parallel to
each other.
[0088] In some embodiments, the devices of FIGS. 22 and 23 have one
or more of the following properties. The thickness of each of the
first and second magnetic layers may be less than 5 nm or even 3
nm, whereas the thickness of the first multi-layered structure may
be in the range of 3 to 11 .ANG.. In some embodiments, the first
multi-layered structure has a thickness of at least eight Angstroms
and not more than ten Angstroms. The first and second magnetic
layers include Heusler compounds (optionally doped with Co)
independently selected from the group consisting of Mn.sub.3.1-xGe,
Mn.sub.3.1-xSn, and Mn.sub.3.1-xSb (with x being in the range from
0 to 1.1 in the case of Mn.sub.3.1-xSb, and with x being in the
range from 0 to 0.6 for Mn.sub.3.1-xGe and Mn.sub.3.1-xSn). In some
embodiments, the first and/or the second magnetic layers may
include a ternary Heusler compound, such as
Mn.sub.3.1-xCo.sub.1.1-ySn, in which x.ltoreq.1.2 and y.ltoreq.1.0.
In some embodiments, the first and/or the second magnetic layers
may include L1.sub.0 compounds such as MnAl alloys, MnGa alloys,
MnSn alloys, MnGe alloys, and FeAl alloys.
[0089] In some embodiments of the structures shown in FIGS. 22 and
23, at least one of E and E' is an AlGe alloy and/or an AlGa alloy.
Alternatively, at least one of E and E' includes an alloy selected
from the group consisting of AlSn, AlGe, AlGaGe, AlGaSn, AlGeSn,
and AlGaGeSn. Also, a seed layer may be used advantageously over
the substrate. The devices of FIGS. 22 and 23 may be used as memory
elements, for example, as part of a racetrack memory device.
[0090] The structures described above in FIGS. 22 and 23 can also
form part of an MRAM element. Specifically, when additional
components are introduced, such as those shown in FIG. 24, an MRAM
element may be formed, in which current passes through the tunnel
barrier and adjoining elements, in turn. As with MRAM memory
elements generally, a tunnel barrier (or other nonmagnetic layer)
is situated between two magnetic electrodes, one of which has a
fixed magnetic moment and the other of which has a magnetic moment
that is switchable, thereby permitting the recording and erasing of
data. Unlike conventional MRAM elements, however, the magnetic
layers of FIG. 24 having fixed magnetic moments (e.g. within the
SAF pinning layer) each comprise a Heusler compound layer and are
separated by a non-magnetic spacer. The Heusler layers, which may
be either ferro- or ferri-magnetic, overlie a templating layer
which in turn overlies a substrate; the Heusler layers have their
respective magnetic moments oriented perpendicular to the layer
plane (or in plane). In some embodiments, a Heusler based SAF
structure may be used as a storage layer. An optional seed layer
may be interposed between the substrate and the templating layer.
An optional polarization layer may be used to increase performance,
which may include Fe, a CoFe alloy, and/or Co.sub.2MnSi. The tunnel
barrier may be MgO(001), although other (001)-oriented tunnel
barriers may be used, such as CaO and LiF. Alternatively,
MgAl.sub.2O.sub.4 can be used as a tunnel barrier; its lattice
spacing can be selected by controlling the Mg--Al composition to
result in better lattice matching with the Heusler compounds (e.g.,
the composition of this tunnel barrier can be represented as
Mg.sub.1-zAl.sub.2+(2/3)zO.sub.4, wherein -0.5<z<0.5). The
magnetic electrode overlying the tunnel barrier may be
advantageously switchable and may comprise Fe, a CoFe alloy, and/or
a CoFeB alloy, for example. The capping layer may include Mo, W,
Ta, Ru, or a combination thereof. Current may be induced by
applying a voltage between the two magnetic electrodes, which are
separated by the tunnel barrier.
[0091] The various layers described herein may be deposited through
any one or more of various methods, including magnetron sputtering,
electrodeposition, ion beam sputtering, atomic layer deposition,
chemical vapor deposition, and thermal evaporation.
[0092] The invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative and not restrictive. The scope of the invention is
therefore indicated by the appended claims rather than the
foregoing description. All changes within the meaning and range of
equivalency of the claims are to be embraced within that scope.
TABLE-US-00001 TABLE 1 Target to Substrate Target Material Distance
Ir-RBS (%) Al-RBS (%) IrAl 135 mm 52.5 47.5 IrAl 125 mm 49.9 50.1
IrAl 120 mm 49.5 50.5 IrAl 113 mm 46.6 53.4
[0093] Although the foregoing embodiments have been described in
some detail for purposes of clarity of understanding, the invention
is not limited to the details provided. There are many alternative
ways of implementing the invention. The disclosed embodiments are
illustrative and not restrictive.
* * * * *